I've actually found that a gut hook does not need to be all that sharp to work well and as a matter of fact I believe that a gut hook can easily be too sharp. The very nature of the hook makes it cut. If you think about it the hook is already under the skin, lifting it away from the meat. The sharp part of the hook has the skin trapped and the action of pulling or dragging it through the skin brings quite a bit of cuting force to the only location that will cut. If the edge is too sharp you can easily dig deeper than intended or pull the hook completely out of the skin requiring you to relocate it and basicly start again. The big game gut hook is not a delicate piece of equipement and it is not intended for fine work. Its a quick and dirty method of splitting hide so you can get down to the real work of skinning.
Edit: I have also used a Dremel tool with on of those long eraser like polishing tools. This works well. Another thing to consider is a plane old chain saw file. THe teeth on these files are fine enough to do a good job.
